Method and device for controlling the distance from a vehicle to a preceding vehicle

ABSTRACT

A method for controlling the distance of a vehicle (F) to a vehicle (P) traveling ahead, in which the distance Dist and the relative velocity of the vehicle (P) traveling ahead are measured and the distance, in a distance control mode, is controlled by accelerating or decelerating the vehicle (F) to a preestablished setpoint distance Dsoll, wherein the deceleration permitted by the distance control process is limited and, in situations in which the setpoint distance Dsoll cannot be maintained at this limited deceleration, the transition is made from the distance control process to a process limiting the distance to a minimum distance Dmin which is smaller than the setpoint distance, and the vehicle (F), after reaching the minimum distance, is further decelerated, so that the distance once again increases to the setpoint distance.

FIELD OF THE INVENTION

[0001] The present invention relates to a method for controlling thedistance of a vehicle to a vehicle traveling ahead, in which thedistance and the relative velocity of the vehicle traveling ahead aremeasured, and the distance is controlled in a proximity-control mode byaccelerating or decelerating the vehicle to a preestablished setpointdistance.

BACKGROUND INFORMATION

[0002] Methods and devices of this type are referred to by the name“adaptive road speed controller” as well as by the abbreviation “ACC”(Adaptive Cruise Control), and they are described, by way of example, inthe article, “Adaptive Cruise Control—System Aspects and DevelopmentTrends” by Winner, Witte, et al., published in the SAE 96, Detroit, pp.26-29, February 1996, Paper No. 961010. Specific aspects of systems ofthis type are described in German Published Patent Application No. 19627 727, German Published Patent Application No. 196 730 245, and GermanPublished Patent Application No. 196 40 694.

[0003] In conventional systems, proximity measurement is usuallyperformed by a radar system, which on the basis of the Doppler effectalso makes possible a direct measurement of the relative velocity, sothat the control system is capable of reacting immediately to measuredchanges in the velocity of the vehicle traveling ahead.

[0004] The setpoint distance, upon which the control is based,corresponds to the safety distance to be maintained in column or convoytravel among vehicles traveling one after the other, and it isdimensioned so that, even in the case of longer vehicle columns andtaking into account the reaction times of the drivers involved, rear-endcollisions do not result if one vehicle has to brake abruptly due to anunexpected obstacle. This safety distance is a function of velocity andis therefore advantageously determined indirectly by a so-calledsetpoint time gap, which corresponds to the temporal interval in whichvehicles pass the same point one after the other. In an ideal pursuit,the path-time curve of the pursuing vehicle is then the precise image,displaced by the setpoint time gap, of the path-time curve of thevehicle traveling ahead. The same also applies to the velocity-timecurve as well as to the acceleration-time curve, it also being possiblefor the accelerations to have negative values (the deceleration of thevehicle is defined as the amount of negative acceleration).

[0005] In practice, inevitable control delays and differences in thevehicle characteristics (acceleration capacity) result in the path-timecurve of the pursuing vehicle deviating somewhat from the correspondingcurve of the vehicle traveling ahead. Deviations of this type are to acertain extent entirely desirable because they result in somewhatsmoothing out speed fluctuations of an “unsteady” vehicle travelingahead. This smoothing effect can also be strengthened in a controlledmanner, for example, by having the accelerations of the vehicletraveling ahead, which are filtered by a low-pass characteristic,accessed by the control system.

[0006] An object of the present invention, in a proximity control systemof this type, is to improve the comfort and the feeling of safety forthe driver and the vehicle occupants.

SUMMARY

[0007] This objective is achieved according to the present invention asdescribed herein.

[0008] In the method according to the present invention, thedeceleration permitted in the proximity control process is limited, andin situations in which the setpoint distance cannot be maintained on thebasis of this limited deceleration, the proximity controlling processtransitions to a process of limiting the proximity to a minimum distancewhich is smaller than the setpoint distance, and after the minimumdistance is reached, the vehicle is further decelerated so that thedistance again increases to the setpoint distance.

[0009] If the own vehicle approaches at high speed the more slowlymoving vehicle traveling ahead, or if, during pursuit, the vehicletraveling ahead is suddenly decelerated, then the method according tothe present invention brings about the result that the following vehicletemporarily “dips” into the setpoint distance and then falls back untilthe setpoint distance is once again attained. In this manner, it may beavoided that the comfort and the feeling of safety are impaired byextreme vehicle decelerations. This dipping strategy, in the methodaccording to the present invention, corresponds to the intuitivebehavior of an experienced automobile driver. As a result of the presentinvention, the behavior of the control system is therefore brought tomore closely approximate the natural behavior of a human automobiledriver, and in the process irritations are avoided which may otherwiseresult from the different behavior of the automatic control system. Thetemporary undershooting of the setpoint distance is unobjectionable fromthe safety technical point of view because it is only brief, and theassumption may be made that the driver is very alert as a result of thebraking. In the traffic-technical sense, the method according to thepresent invention has the effect that velocity fluctuations or abruptvelocity changes are far more powerfully cushioned than would bepossible using a simple proximity control system. It is conventionalthat velocity fluctuations of this type, especially in heavy trafficbuildups on a highway, may be amplified in a regressive wave and mayfinally lead to a traffic jam. In this regard, the present inventionalso contributes to the flow of traffic and therefore ultimately totraffic safety.

[0010] The minimum distance to the vehicle traveling ahead, which is notsupposed to be undershot even in a dipping process, may be described bya time gap, which is designated as the dipping time gap and which issmaller than the setpoint time gap. The minimum distance is then theproduct of dipping time gap and velocity of the own (following) vehicle.The difference between the actual distance and the minimum distance isthe deceleration distance (measured as the relative distance between thevehicles), within which, in response to dipping, the relative velocitybetween the vehicles may be reduced to zero. From this decelerationdistance and the known instantaneous relative velocity, an accelerationvalue may be calculated using the path-time law for a uniformlyaccelerated motion, the acceleration value assuring that the relativevelocity is actually reduced within the deceleration distance. However,this only applies under the assumption that the vehicle traveling aheadmaintains its velocity at a constant value. If this is not the case, thecalculated acceleration value may also have added to it the—ifnecessary, appropriately filtered—acceleration of the vehicle travelingahead. On the basis of the acceleration obtained in this manner, it ispossible to control the dipping process so that the minimum distance isnot undershot, and the acceleration of the own vehicle in its amountremains as small as possible.

[0011] In principle, it is sufficient to determine the decelerationdistance and the acceleration derived therefrom only once at thebeginning of the dipping process and then, in the further course of thedipping process, to take into account only the accelerations of thevehicle traveling ahead. The velocity changes of the vehicle travelingahead, however, may be even further cushioned if the decelerationdistance and the acceleration derived therefrom are also continuallyactualized during the dipping process. However, to prevent thedeceleration distance from becoming excessively small or from decliningto zero, which may result in unrealistically high deceleration values,the deceleration distance in this case may be limited to a positiveminimum value. In determining the deceleration distance, abrupttransitions may be avoided by performing interpolations in a proximityrange below the setpoint distance, between the lower threshold value andthe theoretical deceleration distance, which is yielded by the actualdistance, the instantaneous vehicle velocity, and the setpoint time gap.

[0012] When, at the end of the dipping phase, the minimum distance tothe vehicle traveling ahead has been attained, then the own vehicle maybe further decelerated so that the distance again grows to the setpointdistance. This may be achieved using control technology by basing thecalculation of the necessary acceleration from the deceleration distanceand the relative velocity not on the actual relative velocity but ratherby adding to this relative velocity an appropriate return velocity. Thesystem behaves as if the velocity of the vehicle traveling ahead issmaller by the return velocity than the actual velocity. This has theconsequence that the deceleration of the own vehicle does not end in arelative velocity of zero but rather ends in a positive relativevelocity corresponding to the return velocity, so that the own vehicleagain falls back to the setpoint distance.

[0013] When the setpoint distance is reached, it is possible to switchback to the normal proximity control system. However, with reference toa gentle transition between the different control modes, it is possiblethrough additive superimposition to create a setpoint value from theacceleration values that are generated from the proximity controlprocess and from the proximity limiting process, the setpoint value thenbeing supplied to the engine control system. In the context of theproximity control process, in this case the value range of thepermissible setpoint accelerations is limited so that only setpointaccelerations above a predetermined negative threshold value aregenerated, whereas in the proximity limiting process the value range ofthe accelerations is limited by an upper threshold value, for example,zero. By adding these setpoint values, the result is then a fluidtransition between proximity limiting and proximity control.

[0014] Further advantages are yielded from the following descriptions ofexample embodiments.

[0015] The present invention is described below in greater detail on thebasis of the example embodiment illustrated in the Figures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a block diagram of a control unit for controlling thevelocity of a vehicle.

[0017]FIG. 2 is a path-time diagram of the vehicle and of a vehicletraveling ahead.

[0018]FIG. 3 is, in a time diagram corresponding to that illustrated inFIG. 2, the change in the distance between the vehicle and the vehicletraveling ahead.

[0019]FIG. 4 is a diagram illustrating the relationship between theactual distance of the vehicle traveling ahead and a decelerationdistance taken as a basis for the distance limiting process.

[0020]FIG. 5 is a block diagram of the system for proximity control andproximity limiting.

DETAILED DESCRIPTION

[0021]FIG. 1, in the form of a block diagram, illustrates a control unit10 for an adaptive road speed controller of a motor vehicle, forexample, a passenger car. Control unit 10 includes an input circuit 12,at least one microcomputer 14, and an output circuit 16, which areconnected to each other for data exchange by a communications system 18.

[0022] A velocity measuring device 20 is configured to measure thevelocity of the vehicle, an operating element 22 that may be actuated bythe driver and that functions, inter alia, for inputting the setpointvelocity desired by the driver, and a distance measuring device 24,e.g., a radar device, supply input signals to input circuit 12.Additionally, input circuit 12 receives signals from further measuringdevices 26, 28, which are configured to measure further operatingvariables of the vehicle, which are used in the adaptive road speedcontrol system. Examples of this are the steering angle, the transverseacceleration, etc.

[0023] Microcomputer 14 evaluates the data that are input via inputcircuit 12 in the context of the adaptive road speed controller and, viaoutput circuit 16, it drives a control device 30, for example, anelectronic engine control device, which, for example, by influencing thethrottle valve position, the ignition, etc., of the vehicle engine,determines the driving power and therefore ultimately the (positive ornegative) acceleration of the vehicle.

[0024] Microcomputer 14 periodically executes a program, whichcalculates the setpoint acceleration (or deceleration) to be output ineach case to control device 30. If no vehicle traveling ahead is locatedusing the radar device, then a control process is performed to arrive atthe setpoint velocity that is input by the driver. On the other hand, ifthe presence of a vehicle traveling ahead is established, then, usingthe radar device, its distance Dist and relative velocity Vrel aremeasured, and a control process is performed to arrive atvelocity-dependant setpoint distance Dsoll, which corresponds to therequired safety distance between the vehicles. This case is illustratedin FIG. 2 in the form of a path-time diagram.

[0025] Curve F, which is illustrated in FIG. 2 in thick, solid lines,represents the path-time curve of the vehicle, which is provided withthe adaptive road speed controller. Curve P represents a vehicletraveling ahead. (Reference characters F and P are used hereinafter bothfor the curves as well as for the vehicles represented by them). Up totime point t1, vehicle P travels at a constant velocity, and thevelocity of vehicle F is regulated in the context of proximity controlso that it follows vehicle P at the same velocity at a specific distanceDist. Distance Dist corresponds to the setpoint distance appropriate tothe velocity in question, and it is equal to the product of a setpointtime gap Ts and velocity V of vehicle F.

[0026] In the illustrated example, it is assumed for the sake ofsimplicity that vehicle P traveling ahead is abruptly braked at timepoint t1 and then continues to travel at a slower speed. If the temporalinterval between vehicles P and F were always precisely equal tosetpoint time gap Ts, then the associated path-time curve for vehicle Fwould be represented by curve F′, indicated by solid lines, whichderives from curve P as a result of a parallel shift of Ts. From thereduced velocity of the two vehicles, a correspondingly smaller setpointdistance Dsoll is then generated on the basis of same setpoint time gapTs. Therefore, vehicle F may, within a limited time, reduce its velocityfrom the original velocity to the new velocity of the vehicle travelingahead, so that it may follow vehicle P at new distance Dsoll. However,because Dsoll is smaller than previous distance Dist, vehicle F does notneed to abruptly brake, as is indicated by curve F′, but rather thedeceleration may occur somewhat more gently, as is indicated by thethick dotted line curve F″. This curve guarantees that (new) setpointdistance Dsoll will not be undershot at any time point. However, forthis purpose, a relatively strong deceleration of vehicle F is stillrequired, and this deceleration is often experienced by the vehicleoccupants as disturbing or at least as comfort reducing. Therefore, inthe method according to the present invention, the vehicle is only moreweakly decelerated, as is indicated by curve F. In this context, it isconsciously accepted that the distance between vehicles F and P willtemporarily fall below setpoint distance Dsoll. This means thatfollowing vehicle F dips temporarily into the setpoint distance ofvehicle P traveling ahead and then only slowly returns once again tosetpoint distance Dsoll. In this context, of course, the depth of thedip may be limited so that between vehicles F and P a safe minimumdistance Dmin is always maintained. This minimum distance Dmin is theproduct of velocity V of vehicle F and a dip time gap Te, which issmaller than setpoint time gap Ts.

[0027] Curve G illustrated in FIG. 2 represents a fictitious vehicle,which follows vehicle P to time point t2 at a temporal deceleration thatcorresponds to dip time gap Te. From time point t2 on, however, curve Gis flatter than curves P and F′. This means that the velocity offictitious vehicle G, from time point t2 on, is smaller in the absolutesense than the velocity of vehicle P, so that the spatial and temporalinterval by time t4 is again increased to setpoint distance Dsoll andsetpoint time gap Ts. The velocity of vehicle F is regulated in thecontext of the dip strategy so that the associated path-time curvetouches curve G but does not intersect it. At time point t3, curve Gforms a tangent on curve F. From this time point on, vehicle F no longerneeds to be decelerated, but rather it may travel further at thatessentially constant velocity until at time point t4 setpoint distanceDsoll is again established, and the regular proximity control processmay again be undertaken.

[0028] In FIG. 3, the same sequence is illustrated as in FIG. 2 in theform of a distance-time diagram. Curve f indicates the distance betweenvehicles P and F. Curve f′ illustrates a change in the distance, whichcorresponds to curve F′ illustrated in FIG. 2, and curve f″ represents achange in the distance corresponding to curve F″ illustrated in FIG. 2.Curve F″ is one branch of a parabola, the lower apex of which is atDsoll. The segment of curve f between t1 and t3 is one part of aparabola, the lower apex of which is approximately at minimum distanceDmin and which at time point t3 has a slight positive slope, whichindicates the increase of the vehicle distance corresponding to thereturn velocity. The vehicle distance at time point t3 does notprecisely conform with the minimum distance Dmin in the strictest senseof the word, but this difference in practice is not significant.

[0029] Deceleration distance Dv, within which the velocity of vehicle Fdeclines from the beginning value at time t1 to the target velocity(velocity of vehicle traveling ahead P minus the return velocity) attime t3, corresponds in FIG. 3 roughly to the difference between Distand Dmin and is therefore significantly greater than the differencebetween Dist and Dsoll. The difference between Dsoll and Dmincorresponds to the depth of the dip, by which the vehicle dips into thesetpoint distance.

[0030] Acceleration a0 of the vehicle, corresponding to curve Fillustrated in FIG. 2, is a function of deceleration distance Dv,relative velocity Vrel between the two vehicles, and desired returnvelocity Vrück. Required velocity change dV is equal to Vrel−Vrück. Inthis context, dV and Vrel are negative at least at the beginning,whereas for Vrück a positive value is always selected. The followingthen applies:

a0=sign(dV)dV2/2Dv.  (1)

[0031] If vehicle F between t1 and t3 constantly maintains thisacceleration, then the distance to vehicle P traveling ahead has thecurve indicated by curve f illustrated in FIG. 3, assuming that thevelocity of vehicle P traveling ahead does not change. If vehicle Ptraveling ahead experiences an acceleration ap within this time, thenthe acceleration of vehicle F is corrected as follows:

ac=a0+ap.  (2)

[0032] From equation (1), it may be seen that Dv is not permitted to bezero, because otherwise in calculating the acceleration there would haveto be a division by zero. For this reason, Dv is not always calculatedin accordance with the equation

Dv=Dist−Dmin=Dist−VTe,  (3)

[0033] but rather in accordance with the equation which is representedby curve D illustrated in FIG. 4. Equation (3) therefore only applies tothe case in which Dist is larger than or equal to Dsoll. Dv in any caseis greater than a minimum value Dvmin (for example, 2 meters). If Distis smaller then Dsoll, then Dv continually decreases to minimum valueDvmin. Dv is generally calculated in accordance with the formula:

Dv=MAX(Dvmin, Dist(Dsoll−Dmin)/Dsoll, Dist−Dmin).  (4)

[0034]FIG. 5 in a block diagram illustrates the essential functionalparts of a proximity controller and of a proximity limiter, which areimplemented using appropriate programs executed in microcomputer 14.

[0035] A proximity-controller part 32, which is responsible for thenormal proximity control process in response to moderate changes in thevelocity of the vehicle traveling ahead, includes a characteristicsfield 34, a multiplication member 36, and a limiting part 38.Characteristics field 34 receives as input signals setpoint distanceDsoll, measured vehicle distance Dist, and measured relative velocityVrel, and from these input quantities it ascertains a setpoint velocitychange DV. Using multiplication member 36, DV is multiplied by aregulating amplifier 1/t, by the inverse value of a time constant t, sothat an acceleration value is obtained as the result. This accelerationvalue is limited in limiting part 38 to values above a specific(negative) threshold acceleration amin. Limiting part 38 delivers anacceleration setpoint value ar, as an output signal for the proximitycontrol process.

[0036] Due to the effect of limiting part 38, no vehicle decelerationsthat are greater than the amount of amin are possible within the contextof this proximity control process. In an abrupt change in the velocityof the vehicle traveling ahead, as in the case of the example discussedin conjunction with FIG. 2, controller part 32 is therefore not able tomaintain setpoint distance Dsoll. In this case, a distance limitingprocess occurs using a distance limiting part 40. This distance limitingpart 40 includes a characteristics field or a calculation module 42, anaddition member 44, and a further limiting part 46. First, calculationmodule 42 calculates from the measured data (generally negative)velocity change dV (=Vrel−Vrück) and, on the basis of the equationillustrated in FIG. 4, deceleration distance Dv, and then from this itcalculates, in accordance with the equation (1), uncorrectedacceleration a0. Measured and, if necessary, appropriately filtered ownacceleration ap of the vehicle traveling ahead is added thereto inaddition member 44, so that corrected acceleration ac is supplied tolimiting part 46. In limiting part 46, corrected acceleration value acis limited on the up side by amin and, at the same time, is increased by−amin, so that received acceleration setpoint value ab, for the distancelimiting process, may only take on negative values. Accelerationsetpoint values ar and ab from proximity controller part 32 and fromproximity limiting part 40 are then added in an addition member 48, andthe sum, obtained as setpoint value a, is finally supplied to anotherreturn limiter 50, which dampens abrupt changes in the acceleration soas to improve the driving comfort.

[0037] In the example illustrated in FIGS. 2 or 3, distance controllerpart 32 is essentially active until time point t1, whereas distancelimiting part 40 supplies in any case a negligible contribution tofinally-output setpoint acceleration a, because the relative velocity isclose to zero and therefore velocity change dv is very small. Becausereturn velocity Vrück is only required to reestablish the setpointdistance after a dipping process, dV may always be equal to Vrel if Distis greater than Dsoll. If at time t1 the vehicle traveling ahead isbraked, then the setpoint acceleration, or deceleration ar, which may berepresented by the controller part, is no longer sufficient to maintainthe setpoint distance. The setpoint acceleration is then determined byoutput value ab of the distance limiting part, so that vehicle F in themanner indicated in FIGS. 2 and 3 dips into the setpoint distance. Inthis context, the values of dV and Dv are continually adjusted. Becauseactual distance Dist is smaller than Dsoll, then for determining Dv themore planar part of curve D illustrated in FIG. 4 is effective. Thevelocity of the vehicle gradually approaches the target velocity, withthe result that setpoint acceleration ab, which is output by distancelimiting part 40, in its amount again approaches zero, until finallydistance controller part 32 again dominates and, for example, from timepoint t4 the proximity controlling process is continued.

[0038] In FIG. 2, as an example, the case is illustrated that thevelocity of vehicle P traveling ahead at a specific time point t1abruptly decreases. However, the same dipping strategy is also followedin the case in which vehicle P traveling ahead from the beginning has arelatively low constant velocity, as illustrated in FIG. 2 is indicatedby the dot-dash curve P′, and following vehicle F travels at a highervelocity until at time point t1 the vehicle traveling ahead is locatedby the radar system.

[0039] Whereas, in the example embodiment described, for determining thedepth of the dip a constant dip time gap is assumed, in other exampleembodiments it is also possible to use other characteristic quantitiesas a measure for the depth of the dip. For example, a constant dipdistance Dmin is preestablished, or the dip distance may be selected asproportional to the setpoint distance. Similarly, it is also possible tovary return velocity Vrück in accordance with the situation.

What is claimed is:
 1. A method for controlling the distance of avehicle (F) to a vehicle (P) traveling ahead, in which the distance Distand the relative velocity of the vehicle (P) traveling ahead aremeasured and the distance, in a distance control mode, is controlled byaccelerating or decelerating the vehicle (F) to a preestablishedsetpoint distance Dsoll, wherein the deceleration (−ar) permitted by thedistance control process is limited and, in situations in which thesetpoint distance Dsoll cannot be maintained at this limiteddeceleration, the transition is made from the distance control processto a process limiting the distance to a minimum distance Dmin which issmaller than the setpoint distance, and the vehicle (F), after reachingthe minimum distance, is further decelerated, so that the distance onceagain increases to the setpoint distance.
 2. The method as recited inclaim 1, wherein, in the context of the distance limiting process, anacceleration a0 of the vehicle is calculated, the acceleration beingnecessary so that, during the time in which the distance Dist betweenthe vehicles is reduced by a preestablished deceleration distance Dv,which is greater than the difference between Dist and Dsoll, a velocitychange dV arises which in its amount corresponds at least to therelative velocity Vrel, and the own acceleration ap of the vehicle (P)traveling ahead is added to this acceleration a0.
 3. The method asrecited in claim 2, wherein the deceleration distance Dv is continuallyvaried as a function of the distance Dist and of the velocity of thevehicle (F).
 4. The method as recited in claim 3, wherein thedeceleration distance Dv is equal to the difference between theinstantaneous distance Dist and the minimum distance Dmin, as long asthe Dist is greater than the setpoint distance Dsoll.
 5. The method asrecited in claim 4, wherein the deceleration distance Dv is alwaysgreater than a positive minimum value Dvmin.
 6. The method as recited inclaim 5, wherein the deceleration distance Dv is calculated according tothe following formula: Dv=MAX (Dvmin, Dist (Dsoll−Dmin)/Dsoll,Dist−Dmin).
 7. The method as recited in one of claims 2 through 6,wherein, at least as long as the instantaneous distance Dist is smallerthan the setpoint distance Dsoll, the velocity change dV is determinedby subtracting from the relative velocity Vrel a preestablished positivereturn velocity Vrück.
 8. The method as recited in one of the precedingclaims, wherein the setpoint acceleration a of the vehicle is determinedby so linking a setpoint acceleration ar, ascertained in the context ofthe distance control process, to a setpoint acceleration ab, ascertainedin the context of the distance limiting process, the setpointacceleration ar, ascertained in the context of the distance controlprocess, dominates above a preestablished negative thresholdacceleration amin, and the setpoint acceleration ab, ascertained in thecontext of the distance limiting, dominates below this thresholdacceleration.
 9. A device for controlling the distance of a vehicle (F)to a vehicle (P) traveling ahead, having a device (24) for measuring thedistance and the relative velocity of the vehicle traveling ahead and adata-processing device (14) having a distance control part (32), whichdetermines a setpoint acceleration (ar) for controlling the distance tothe vehicle traveling ahead at a preestablished setpoint distance(Dsoll), wherein the setpoint acceleration (ar) output by the distancecontrol part (32) is limited to accelerations above a preestablishednegative limiting value (amin), and the data processing device (14) hasa distance limiting part (40), which when the setpoint distance (Dsoll)is undershot, becomes operative and determines a setpoint acceleration(ab), as a result of which the distance to the vehicle traveling aheadis limited to a minimum distance (Dmin) which is smaller than thesetpoint distance, and as a result of which the velocity of the vehicle(F) is returned to a value which is smaller than the velocity of thevehicle (P) traveling ahead.